linux-kernel - Re: [x86] do_arch_prctl

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Date:	Tue, 18 Nov 2008 18:35:10 +0100
From:	Eric Lacombe <goretux@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: [x86] do_arch_prctl - bug?

In fact, if what I thought from that function was ok, we will have in fact the 
order of the two following lines inversed :
...
  load_gs_index(0);
  ret = checking_wrmsrl(MSR_KERNEL_GS_BASE, addr);
...

So that we would have :
...
  ret = checking_wrmsrl(MSR_KERNEL_GS_BASE, addr);
  load_gs_index(0);
...

Regards,

	Eric

Le mardi 18 novembre 2008 18:20:03 Eric Lacombe, vous avez écrit :
> Le mardi 18 novembre 2008 15:45:56, vous avez écrit :
> > On Tue, 18 Nov 2008 15:33:32 +0100
> >
> > Eric Lacombe <goretux@...il.com> wrote:
> > > Hello,
> > >
> > > I would like to know why the ARCH_SET_GS action of sys_arch_prctl,
> > > write the MSR MSR_KERNEL_GS_BASE and not the MSR MSR_GS_BASE when the
> > > variable "doit" equals 1? Is that a bug?
> >
> > I don't think it is.
> > The trick is that we use "swapgs" on entering/leaving the kernel, and
> > that will "swap" gs with the MSR, so when we return to userspace, GS
> > gets loaded from the MSR_KERNEL_GS_BASE ...
>
> Yeah when we enter the kernel swapgs is used, so the MSR_GS_BASE is
> switched with the MSR_KERNEL_GS_BASE.
>
> In fact, what I certainly misunderstand is why load_gs_index use swapgs
> inside.
> From that function, I trust that only when gs is loaded, its hidden part is
> loaded with the MSR_GS_BASE.
>
> ENTRY(native_load_gs_index)
>         CFI_STARTPROC
>         pushf
>         CFI_ADJUST_CFA_OFFSET 8
>         DISABLE_INTERRUPTS(CLBR_ANY | ~(CLBR_RDI))
>         SWAPGS
> gs_change:
>         movl %edi,%gs
> 2:      mfence          /* workaround */
>         SWAPGS
>         popf
>         CFI_ADJUST_CFA_OFFSET -8
>         ret
>         CFI_ENDPROC
> ENDPROC(native_load_gs_index)
>
> Regards,
>
> 	Eric

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